Surface Plasmon Resonance (SPR) refers to the collective oscillation of valence electrons of a solid upon being irradiated by photons generated by incident electromagnetic (EM) radiation. In particular, SPR refers to the resonance between the frequency of the EM photons and the natural frequency of the valence electrons which results in the resonant oscillation of the valence electrons. Where the SPR occurs in nano-sized structures, it is termed “Localized Surface. Plasmon Resonance” (LSPR). Notably, the SPR phenomena can be used for the detection of molecules which become adsorbed onto the surface of a said (e.g., a plasmon-active metal).
Accordingly, methods of fabricating substrates supporting a layer of plasmon-active metal film are of great interest, particularly in the field of chemical sensors and biosensors. For instance, it is known that an ordered array of nanostructures with gold films can directly convert incident light into surface plasmons (SPs), which can lead to enhanced optical transmission or extraordinary optical transmission (EOT). In some studies, nanohole arrays in a gold film have been used as biomolecular sensors to detect and study surface-binding interactions using the EOT phenomenon.
The key advantages of nanohole array-based plasmonic sensors over conventional prism- or grating-based SPR sensors are (1) the elimination of the need for a bulky prism, (2) simple light coupling instrumentation, (3) use of small reagent volumes, (4) high sensitivity, and (5) easy integration, miniaturization and multiplexing.
However, a significant obstacle faced by current nanohole array-based plasmonic sensors relates to the lack of fabrication techniques that are suitable for large output, large area (centimeter-scale) and low-cost production of plasmonic substrates with ordered nanoholes as sensing elements.
Nanohole arrays are commonly fabricated by beam-based techniques such as e-beam lithography (EEL) and focused ion beam (FIB). Despite having high resolution and control, these beam techniques are generally time-consuming and are limited by small throughput due to the serial nature of such lithography techniques. Additionally, these lithography techniques require sophisticated infrastructure and incur high capital costs. Accordingly, it is difficult to achieve a high throughput with large area fabrication using techniques such as EBL or FIB. As such, these techniques are not commercially feasible for large scale implementation.
Alternatively, other techniques such as nanoimprint (NIL) and nanosphere lithography (NSL) or colloidal lithography techniques have also been considered in the state of the art. NIL is capable of producing high-quality nanoholes on a wafer-scale, but requires multiple steps and expensive nano-imprinting equipment.
On the other hand, nanosphere lithography (NSL) employs self-assembled polystyrene/silica nanospheres as a lithographic mask to fabricate sub-wavelength patterns However, the NSL technique faces significant technical difficulty in achieving defect-free self-assembled colloidal spheres, which are typically larger than 100 μm. Hence, it follows that the NSL technique would encounter even greater difficulty in achieving a defect-free self-assembled monolayer of nano-sized structures.
Accordingly, there is a need to provide a method for providing e polymer substrate supporting a plasmon-active metal film that overcomes, or at least ameliorates, one or more of the disadvantages described above.
In particular, there is a need to provide a method for preparing a polymer substrate that can be used for SPR or LSPR applications, which is capable of high throughput, large surface area fabrication and is cost-effective to implement.